Recently, with increasing integration degree of a semiconductor device, dimensions of individual element become finer, and widths of wiring and gate constituting each element also become finer.
A process of transferring an original plate (a mask or a reticle, hereinafter collectively referred to as a mask) to a photosensitive resin to fabricate a wafer is fundamental to production of a semiconductor integrated circuit. The semiconductor integrated circuit is produced by repeating the fundamental process.
An exposure apparatus called a stepper or a scanner is used in the transfer process. In the exposure apparatus, light is used as a transfer light source, a circuit pattern on the reticle is projected onto the wafer while reduced to about one-fourth to about one-fifth. In order to increase the integration degree of the semiconductor integrated circuit, it is necessary to improve resolution performance in the transfer process. Assuming that NA is an aperture factor of an imaging optical system, and that λ is a wavelength of the light source, a resolution dimension is proportional to (λ/NA). Accordingly, higher exposure resolution can be achieved by increasing the aperture factor NA or decreasing the wavelength λ.
Nowadays, an EUV (Extreme Ultra Violet) lithography technology in which EUV light having the short wavelength (λ=13 to 14 nm) is used is being developed. Although the aperture factor NA of EUV lithography is smaller than that of photolithography, the circuit pattern is resolved by about 1.5 times to about 2.0 times of the wavelength in the EUV lithography. Therefore, the resolution dimension of 30 nm or less can be obtained.
Because a large cost is imposed on the production of the semiconductor integrated circuit, it is necessary to improve a yield. A defect of a mask pattern can be cited as one of large factors that degrade the yield. Therefore, in a mask inspection, it is necessary to detect the extremely small pattern defect. Japanese Patent No. 4236825 discloses an inspection apparatus that can detect the fine defect on the mask.
In the mask inspection, the mask is illuminated with the light while moved, and the pattern formed on the mask is imaged with an imaging element. Then, an obtained optical image is compared to a standard image, and a place where a difference between the optical image and the standard image exceeds a threshold is detected as the defect.
A density of the pattern formed on the mask is not constant. Such a region as a memory mat portion of a semiconductor chip, where the pattern density is high, and such a region as a peripheral circuit portion of the semiconductor chip, where the pattern density is low, are mixed in the pattern.
FIG. 1 is a schematic sectional view of the mask, and illustrates a state in which a patterned film 92 such as a chromium (Cr) film is provided on a glass substrate 91. In FIG. 1, a region A is a coarse pattern area and a region B is a dense pattern area. FIG. 2 illustrates a light quantity value of the light incident to a TDI (Time Delay Integration) sensor when the pattern in FIG. 1 is imaged with the TDI sensor along an X-axis direction. As illustrated in FIG. 2, the region A having the low pattern density has the large light quantity value, and is brightly observed on the mask. On the other hand, the region B having the high pattern density has the low light quantity value, and is darkly observed on the mask. Therefore, although the high-contrast image is obtained in the region A, the high-contrast image is not obtained in the region B, but a noise increases to degrade inspection accuracy. On the other hand, when the quantity of light with which the mask is illuminated increases to obtain the high-contrast image in the region B, unfortunately the quantity of light incident to the sensor increases excessively in the region A and the sensor reaches a saturated state.
The similar problem is generated in an EUV mask.
FIG. 3 is a schematic sectional view of the EUV mask. As illustrated in FIG. 3, in the EUV mask, a multilayer film 94 that is made of molybdenum and silicon while constructed with a predetermined number of layers is stacked on a glass substrate 93 to form a reflecting layer. A patterned film 95 is provided on the multilayer film 94. The film 95 is an absorbing layer made of a material having a high absorption coefficient with respect to the EUV light. A buffer film (not illustrated) is provided between the multilayer film 94 and the film 95. The buffer film reduces damage to the multilayer film 94 in patterning the film 95 or correcting the defect.
In FIG. 3, the region A is the coarse pattern area and the region B is the dense pattern area. FIG. 4 illustrates the light quantity value of the light incident to the TDI sensor when the pattern in FIG. 3 is imaged with the TDI sensor along the X-axis direction. As illustrated in FIG. 4, the region A having the low pattern density is brightly observed on the mask, and the region B having the high pattern density is darkly observed on the mask. Therefore, similarly to the example in FIG. 1, there is generated the problem in that the inspection accuracy is degraded in the region B. When the quantity of light with which the mask is illuminated increases, the quantity of light incident to the sensor increases excessively in the region A and the sensor reaches the saturated state.
The EUV mask in FIG. 3 is more noticeable than the mask in FIG. 1 in a light quantity difference between the region A and the region B. The following items are cited as the main reason. That is, 1) the multilayer film 94 is provided on the glass substrate 93, and 2) the pattern in the EUV mask is largely influenced by diffraction because the pattern in the EUV mask is finer than that of the mask in FIG. 1.
Therefore, there is a strong demand to develop the inspection apparatus and inspection method for being able to accurately inspect the mask, particularly the EUV mask. An object of the present invention is to provide an inspection apparatus and an inspection method for being able to accurately inspect both the region having the high pattern density and the region having the low pattern density.
Other challenges and advantages of the present invention are apparent from the following description.